2012 Annual Report 1a.Objectives (from AD-416): Although aphid endosymbionts are known to confer resistance to attack by some parasitoid species, very little is known about the impacts such endosymbionts have on biological control of pest aphids. We will determine whether the aphid endosymbiont HAMITONELLA DEFENSA affects parasitism by APHELINUS species being considered for introduction against pest aphids. 1b.Approach (from AD-416): We will test whether parasitism of APHIS CRACCIVORA by APHELINUS species that are candidates for introduction against soybean aphid is affected by the presences of HAMITONELLA DEFENSA. 3.Progress Report: A previous experiment on the effect of a endosymbiotic bacteria of aphids (HAMITONELLA DEFENSA) on resistance to parasitism by APHELINUS GLYCINIS, a candidate for introduction against soybean aphid, showed that although parasitism was not significantly affected, adult body size of parasitoid progeny was smaller in HAMITONELLA-infected aphids than in HAMITONELLA-free APHIS CRACCIVORA of the same genotype. We carried out an experiment to confirm this result using both APHIS CRACCIVORA and ACYTHROSIPHUM PISUM. We are also testing whether the effects differ for a parasitoid species with a broader host range, APHELINUS CERTUS, which was accidentally, introduce into the United States and is now parasitizing soybean aphid.